As a work vehicle of this type, there is known a bulldozer equipped with a pump absorption torque control unit such as disclosed in Japanese Patent Publication KOKAI No. 10-220359, according to which the power of the engine is transmitted to the right and left sprockets through a torque converter, a transmission, and a differential steering means composed of planetary gear trains etc., whereas the power of a hydraulic motor driven by pressure oil from a hydraulic pump operated by the engine is transmitted to the right and left sprockets through the differential steering means. The pump absorption torque control unit is designed such that when engine speed is high, the absorption torque of the hydraulic pump is increased thereby increasing the work rate and turning performance and when engine speed is low, the absorption torque of the hydraulic pump is reduced thereby ensuring traction ability, engine output recovery performance and the like. That is, in this bulldozer, running load and engine speed are correlated and the absorption torque (absorbed horsepower) of the hydraulic pump is controlled according to engine speed.
The above conventional bulldozer has, however, revealed the following problem. Since engine output torque, which is output from the engine to be input to the transmission through the torque converter, is converted into a desired magnitude of torque according to running load by the torque converter and the transmission to be output to a downstream power transmission mechanism, running load is not correctly reflected in engine speed in some cases. For this reason, optimum control of the absorption torque of the hydraulic pump according to running load cannot be performed. More specifically, for instance, when turning the vehicle with engine speed being reduced through deceleration to decrease vehicle speed, pump absorption torque decreases even though running load is relatively low, so that the flow rate of pressure oil discharged from the hydraulic pump to the hydraulic motor decreases, causing poor turning performance.
The present invention is directed to overcoming such a problem and a primary object of the invention is therefore to provide a work vehicle with a hydrostatic steering system capable of performing optimum control of the absorption torque of a hydraulic pump according to running load, preferentially ensuring turning performance and the speed of an implement when running load is relatively low, and ensuring desired turning performance even when running load is relatively high.